Transcriptional control of osteogenesis in vivo is a complex process involving a large number of genes and phenotype specific as well as general regulatory proteins. Many of these factors, their cognate elements and their functional co-factors are all present in a rate limiting concentration, and must assemble into distinct multi-component regulatory complexes to selectively activate or suppress target genes in response to physiological cues. Null mutation of the Runx2 transcription factor, has shown that it is obligatory for endochondral ossification, osteoblast differentiation and bone formation during embryonic development. However, increased expression of Runx2 in early stage, proliferating osteoblasts is linked with osteopenia land bone fractures. Embryonic and neonatal lethality of mice with targeted disruption of the Runx2 gene precludes characterizing the impact of the encoded regulatory factor on postnatal skeletogenesis, bone remodeling, fracture healing and aging. Our hypothesis that Runx2 is a master switch which contributes to control of bone formation and remodeling of the adult skeleton will be experimentally addressed by regulated postnatal ablation of the Runx2 protein. The goal of this proposal is to generate a conditional mouse model where Runx2 is inactivated in a cell type and tissue-specific manner, at selected embryonic and postnatall stages. In Aim 1, a transgenic mouse carrying LoxP sites flanking exon 8 will be generated for in vivo and ex vivo functional inactivation by Cre recombinase under the control of a regulated promoter. Aim 2 will generate and characterize the inducible and skeletal tissue specific Cre transgenic mouse. Aim 3 will identify the role of Runx2 in bone formation and remodeling of the adult skeleton through phenotype analysis at different ages. Our long-term objective is to use this conditional mouse model to define the Runx2 function during increased bone turnover and fracture healing. These studies will provide insights into the molecular mechanisms governed by Runx2 during osteoblast differentiation that may be translated to novel therapies for congenital and degenerative skeletal diseases, metabolic bone disorders or bone tumors.